Does anyone know how they compute the maximum allowable take-off weight
(and, thus, the useful load) of a Spam Can like we fly? I assume it's
got something to do with minimum climb performance, but what are the
parameters?

I don't think there's a general answer. I think the answer has to be
specific to the model you're asking about. For one, it might be climb
performance, for another the wing spar strength, for another landing gear
strength, for another control authority, etc.

It's one of those bottleneck performance questions where you beef up one
area only to find out that just gets you to the next bottleneck.

I don't think there's anything in the public record that specifies what
the limiting parameter is for given type certificate.

Well, taking our Pathfinder as an example, somehow Piper managed to get a
1400+ pound useful load out of a 235, which is a huge difference from the
180.

True, that's a 30% horsepower increase, and Piper did have to beef up the
sheet metal in some places, but essentially they got the greatest useful
load of any GA 4-seat plane by simply slapping a 6-cylinder engine on the
front of an Archer.

But how did they determine what that useful load was? Did they keep adding
lead ingots until the plane couldn't climb more than "x" feet per minute?
Did they keep landing it at greater weights until the gear failed? You
always hear stories about the guy who "flew out with a moose in the back" --
which implies that useful load is set conservatively, with at least some
margin or fudge-factor built in.

I kick myself for not asking this at the Cherokee Pilots Association fly-in
last year. One of the original Cherokee designers was the keynote speaker,
and I'm sure he could have answered this question. (Chuck Suma, president
of New Piper, is the keynote speaker this year -- so maybe I'll ask him...)
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"